Research Highlights

Cell growth: Signals from the sugar jungle

Bi- (top) and tetra-antennary N-glycans with N-acetylactosamine branches which are binding partners for galectins. For symbol meaning see here.

In addition to their multifaceted cellular functions, N-glycans regulate growth factor and cytokine receptor expression on the cell surface. Galectins are carbohydrate binding proteins expressed on epithelial and immune cells and have multiple binding sites for the terminal lactosamines of N-glycans, thus forming a lattice of N-glycans and galectins that impedes receptor endocytosis and lateral movement. N-glycans are branched, and stable lattice formation requires either many low-branched or a few high-branched (see figure) N-glycans. The medial Golgi pathway is rate-limiting for branching. It involves the sequential transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to the N-glycan core by mannoside GlcNAc transferases (Mgats) such as Mgat5, which adds a fourth GlcNAc antenna to a N-glycan with three branches. De novo UDP-GlcNAc biosynthesis, in turn, is dependent on the availability of key metabolites derived from fatty acid oxidation (acetyl-CoA), amino acid metabolism (glutamine) and nucleotide biosynthesis (UTP).

Previous studies performed by Dennis and Demetriou showed that Mgat5^-/- mice exhibited decreased tumor formation but increased susceptibility to autoimmune diseases when compared to wild-type mice. Furthermore, the authors reported that cytokine receptor signaling strength in tumor cells and their invasiveness are dependent upon Mgat5 expression. In a conceptual study published in Cell, the Dennis and Demetriou groups have further examined the correlation between strength of receptor tyrosine kinase signaling, cellular UDP-GlcNAc concentration and N-glycan structure.

The authors combined computational modeling and validation with cell culture experiments to analyze the influence of the Golgi N-glycosylation pathway on the residency of various surface receptors. They showed that in addition to the biochemical properties of  N-glycan branching and its dependence on nutrient flux to UDP-GlcNAc biosynthesis, the number of N-glycans per glycoprotein receptor is a critical factor for cell surface presence. N-glycan GlcNAc-branching in the cell increases with the availability of the donor substrate UDP-GlcNAc, and this enhances the affinities of glycoproteins for galectin, which in turn supports surface retention of the protein. Growth-promoting receptors (such as the epidermal growth factor receptor) are among the glycoproteins with high numbers of N-glycans, which tend to be bound to galectins, and receptor kinases known to mediate arrest and differentiation (such as the transforming growth factor-b and T cell proliferation limiting receptors) have few N-glycans. Thus, growth-promoting receptors exhibit early and graded increase in cell surface expression, while growth-limiting receptors show a delayed and switch-like response to increasing UDP-GlcNAc concentration. At a sufficiently high GlcNAc concentration, the N-glycan branchedness allows the receptors to form stable associations with the glycoprotein-galectin lattice, which blocks their endocytosis and sensitizes non-malignant cells to cues for growth arrest signaling.

The authors conclude that the N-glycan number and the degree of branching collaborate to regulate the proportion of different types of surface receptors, and thus the sensitivity of the cell to growth and arrest cues. Both groups offer a powerful framework to test this hypothesis in vivo, and have already provided three follow-up studies.

Author: Mirko von Elstermann